Since nutriculture does not employ the soil for cultivating crop plants, there is no replant problem, and it is easy to control cultivation environment, and nutrient and water managements. Further, the nutriculture allows for automation and energy saving, and has been noticed as a cultivation method having high harvest cleanliness and high fertilizer efficiency.
Since a solid medium for nutriculture is immersed in water, water resistance to a certain degree is needed and also water permeability, water retentivity, air-permeability, strength, and the like are required. Conventionally, as a solid medium for nutriculture, rock wools prepared by converging natural stones (mainly basalt) in a fiber shape are known.
For nutriculture, since roots are scattered around in the interior of a solid medium, there may be a need to exchange with a new solid medium. However, when rock wool is used as a solid medium, the rock wool is difficult to recycle. Furthermore, since the rock wool is inorganic material, there is no effective method of disposing of the used rock wool. Presently, as a disposal method, various methods are adopted such as a method of disposal the rock wool as industrial waste and a method of spading the rock wool in small quantities in a field. However, there is a limit in these methods. This calls for a medium capable of maintaining physicality such as water resistance, water permeability, water retentivity, air-permeability, and strength, which are needed as a medium for plant cultivation, sufficiently growing crop plants with a physicochemically stable property, and having a small environment burden.
In order to achieve the above objects, a method of using, as a medium for plant cultivation, a carbide obtained by carbonizing organic waste produced when preparing food, such as brewer's grains is known (see Patent Reference 1). Patent Reference 1 mentions that such a medium has excellent air-permeability, water retentivity, and the like, and has a small environment burden when being produced or disposed. However, it is difficult to put the medium to practical use because it is needed to burn the medium at high temperature (for example, 750 to 850° C.) when obtaining a carbide, the operation is complicated, and energy (costs) for carbonization is increased.
Meanwhile, a medium for plant cultivation using polyvinyl alcohol is also known. For example, Patent Reference 2 mentions that completely saponified type polyvinyl alcohol resin having a degree of saponification of 98 mol % or more is used as a binding agent for charcoal powder. Patent Reference 2 describes a medium in which the completely saponified type polyvinyl alcohol resin having a degree of saponification of 98 mol % or more is hydrophilic but is difficult to be dissolved in water, and when the resin is used as a binding agent for the charcoal powder, it is possible to improve the wettability relative to water on the particle surfaces of the charcoal powder and the resin is not easily destructed when it is immersed in water. However, even when the completely saponified type polyvinyl alcohol resin having a degree of saponification of 98 mol or more is used, the resin is still water soluble polymer. When the medium mentioned in Patent Reference 2 is used for a long period of time, since the polyvinyl alcohol resin is slowly flowed out, it is difficult to grow crop plants for a long period of time.
Further, Patent Reference 3 mentions a nutriculture method using a porous article in a particle shape or chip shape as a medium, in which the porous article is preferably prepared by using, in view of water absorptive and hygroscopic retentions, polyvinyl alcohol as a material. However, as described above, since the polyvinyl alcohol is water soluble polymer, a long-term stability is not sufficiently obtained under a wet condition and in a case of growing crop plants for a long period of time, it is difficult to maintain a shape of the medium. Accordingly, it is not easy to realize the practical use of these products.
In addition, since the polyvinyl alcohol has high affinity with water and high water retentivity, in a case of cultivating crop plants such as a tomato, there is a problem in that water stress is not applied to the crop plants, and thus a sugar content is not increased. In addition, in a case of using polyvinyl alcohol chip as a medium, the chips are fused due to long-term use, and thus air gaps between the chips are reduced, resulting in inhibiting the growth of plant roots. In addition, the chips are fused in a state of gathering the roots of plants therein, and thus it is difficult to remove the roots. Accordingly, there is a problem in that recyclability of the medium is not insufficient.